[cmake] Use CMAKE_CXX_SIMULATE_ID to distinguish MSVC ABI in Findzstd

Instead of unconditionally assuming MSVC-alike behavior for WIN32 that
could negatively impact MSVC, use `CMAKE_CXX_SIMULATE_ID` to detect
Clang using MSVC ABI.  Note that this variable is unset for MSVC itself,
so use it alternatively to MSVC.  Thanks to @isuruf for the suggestion
(conda-forge/llvmdev-feedstock#306 (comment)).

llvm/cmake/modules/Findzstd.cmake

@@ -10,7 +10,7 @@
 # zstd::libzstd_shared
 # zstd::libzstd_static
 
-if(WIN32)
+if(MSVC OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
   set(zstd_STATIC_LIBRARY_SUFFIX "_static\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 else()
   set(zstd_STATIC_LIBRARY_SUFFIX "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
@@ -33,7 +33,7 @@ if(zstd_FOUND)
     set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
   elseif (NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
-    if(WIN32)
+    if(MSVC OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
       include(GNUInstallDirs) # For CMAKE_INSTALL_LIBDIR and friends.
       # IMPORTED_LOCATION is the path to the DLL and IMPORTED_IMPLIB is the "library".
       get_filename_component(zstd_DIRNAME "${zstd_LIBRARY}" DIRECTORY)